This invention relates to cold hearth refining of metals such as titanium alloys which must be completely free of unrefined inclusions and, more particularly, to a new and improved cold hearth arrangement which is especially adapted to prevent contamination of refined metal.
In applications wherein metals such as titanium alloys which have been refined by cold hearth refining are used in aircraft engine parts, for example, the presence of tiny amounts of unrefined inclusions in the refined ingot is severely detrimental. Since such inclusions may, for example, result in fracture and disintegration of aircraft engine parts rotating at very high speed, they should be completely avoided.
In conventional cold hearth refining of metals such as titanium alloys, a water-cooled hearth is supplied with lumps or pieces of titanium sponge or machine turnings of titanium alloy consisting of scrap from the manufacture of titanium alloy parts. This material is introduced by gravity feed at one end of a cooled, elongated hearth in a furnace in which the material is first melted and then refined by energy input from electron beam impingement or plasma torches. The refined molten material is poured from the opposite end of the hearth into a cylindrical mold where it forms a vertically disposed cylindrical ingot that is withdrawn downwardly within the mold as it solidifies.
In conventional cold hearth furnaces used for refining of titanium alloy or the like, in which compacted briquettes and large solid fragments of titanium alloy material are introduced into the melting region, unmelted portions of the alloy material may roll or float downstream from the melting region into the refining region of the hearth so as to contaminate the refined metal in the casting region. Light solids, such as chopped tubing, may also escape complete melting and float into the refining region, causing the same problem.
Heretofore, efforts have been made to separate different portions of the hearth in cold hearth refining systems by mounting physical barriers, such as dams or partitions, in the hearth. In the Hunt U.S. Pat. No. 3,343,828, for example, an elongated flow path is formed in the hearth by providing partial dividers made of graphite which extend alternately from the opposite sides of the hearth to define a serpentine path. This arrangement is not only expensive, but also is incapable of variation without complete reconstruction of the hearth. The Heimerl U.S. Pat. No. 3,748,070 discloses the formation of dams or barriers between one hearth region and another by the provision of cooling bodies at a selected location either within the hearth itself or positioned in contact with the surface of the molten material, causing the molten material to solidify and form a barrier in the immediate vicinity of the cooling body. This arrangement also requires additional hearth structure with a corresponding increase in cost and is incapable of modification without reconstructing the hearth.
Accordingly, it is an object of the present invention to provide a new and improved cold hearth refining arrangement which overcomes the above-mentioned disadvantages of the prior art.
Another object of the invention is to provide a new and improved cold hearth refining arrangement providing a separation between the melting region and the refining region of the hearth which is capable of structural modification without disassembly of the hearth.